1. Field of the Invention
This invention relates to a material metering unit for use in the manufacture of cement or ceramic tiles.
2. Description of the Prior Art
Tile presses are known for manufacturing cement tiles from one or two layers of material. Where two layers are used, in general one of these layers is a semi-liquid layer and the other one is damp or wet and is termed a semi-dry layer. Alternatively, the two layers can both be wet and have differing granularities and/or they can differ in the quantity of material used. Tiles or panels can also be manufactured using a single wet layer only.
Tiles similar to stoneware or for subsequent enamelling can be made using a single cement-free wet layer.
In the manufacture of two layer cement tiles, the semi-liquid layer is first introduced into a mold by an automatic metering unit and the amount of the material in this layer can be readily varied by altering the capacity of the metering or dosage container. The semi-dry layer (whose moisture or humidity can be as high as 10%) is then introduced into the mold, by a loader which completely fills the unoccupied mold space with the semi-dry material. The quantity of material in this second layer can only be varied by changing the height of the mold frames, the material feed system being based upon a complete filling of the mold space. Thus to modify the depth of a product requires the mold frames to be changed but even this does not ensure that the thickness of a finished product is as desired, since the thickness of the product is also dependent on the granularity of the material and the moisture content of the wet layer.
Similar problems of mold filling are also encountered in, for example, the manufacture of ceramic tiles and the like.
It has been previously proposed to overcome the need to change the mold frames, upon a variation in the quantity of material in a mold being required, by using mold frames of increased height and these frames will be only partially filled to a required level whereby to meter the amount of material to be molded. In this previously proposed method, a sieve is adjustably positioned inside the mold and the mold is then filled with material via the sieve which must be vibrated in order for the material to pass therethrough.
The sieve is vibrated for a predetermined time after which vibration is stopped and the sieve is lifted out of the mold. Semi-dry material now fills the mold up to the level at which the sieve was positioned in the mold. However, this method of mold filling with a metered quantity of material has not proved fully satisfactory.
Since the damp semi-dry material does not readily `flow` around inside a mold to give an even distribution of material therein, in addition to metering the semi-dry material, it must also be introduced into the mold in an evenly distributed manner, and known semi-dry material units do not achieve such a material distribution.
Further, with known tile presses it is not possible to use multiple or low mold frames to make thin products since reduction of the thickness of the mold frames reduces quadratically the frame strength. A high frame strength is necessary since the frames must be firmly clamped against the rubber mold bottoms to obtain a perfect seal between the frames and bottoms during pressing to produce "green" tiles.
In metering units incorporating a chest which is arranged to slide across the top of a mold frame to allow material in the chest to fall into and totally fill the mold, the first layer introduced into the mold with a no-too-dense consistency, in order to make it easier for it to lie down or spread thicker, is entrained and piled up in the direction of motion of the loading chest of the second layer. On the resulting molded tiles, there is both a different thickness of the first layer and a different specific pressing and also a smaller total thickness of the tiles in the zone with a smaller thickness of the first layer. Secondly, along the walls of the frame which are transversal with respect to the chest motion, there is some unevenness in the density of the semi-dry material, owing to the compression of the material due to the coaction between the transversal walls of the frame and chest.